The major goal of this study is to gain insight into component cell populations and their interactions in the development of immunologic competence. Studies are focused on the relationships between the lymphoid cells and their surrounding cells and tissues. The investigations mainly use the chicken as an experimental animal with major attention directed to the bursa of Fabricius as an organ for both the differentiation of B lymphocyte competence and an organ functioning in the local defense of the gut. Investigations of the bursa show that a close interrelationship exists between the follicle associated epithelium and the underlying lymphoid follicles. Following cyclophosphamide treatment (used to make chickens B lymphoid cell deficient), it was observed that in addition to a depletion of bursal lymphoid follicles there also was a functionally impaired epithelium. This epithelium is being investigated for its ability to absorb colloids and for ultrastructural changes. Another way in which interaction between cells has been shown was by determining the role macrophages play in the ability of lymphocytes to respond to mitogens. It has now been determined that critical numbers of macrophages must be present for splenic lymphocytes to respond to mitogens. Under experimental conditions both enhancement and inhibition of this response is being investigated.